Don't preallocate based on bsatn length prefix

[joshua-spacetime]

Description of Changes

Fixes an OOM kill in the proptest bsatn_invalid_wont_decode.

bsatn_invalid_wont_decode generates arbitrary invalid bytes, proves validation fails, then still calls full AlgebraicValue::decode. For generated array-like types, decode reads a u32 length prefix, and the generic array visitor then reserves that capacity. But because they're random bytes, this could cause a huge initial allocation which could OOM kill the test process.

Now the visitor reserves a smaller initial capacity instead of assuming the binary input data is well formed.

API and ABI breaking changes

N/A

Expected complexity level and risk

1

Testing

This should fix the flaky spacetimedb-sats Test Suite failures that occasionally end in a SIGKILL.